Iron-based powder for powder metallurgy

ABSTRACT

Flowability-improving particles are adhered to surfaces of iron powder through a binder to provide an iron-based powder for powder metallurgy which has excellent flowability and which is capable of uniformly filling a thin-walled cavity and compaction with high performance of ejection force.

TECHNICAL FIELD

The present invention relates to an iron-based powder suitable for usein powder metallurgy and a method for producing the same.

BACKGROUND ART

Powder metallurgical technology is technology for producing products(sintered compacts) by compaction-molding metal-based powders used aslow materials with a mold and sintering the resultant green compacts.

Powder metallurgical technology is capable of producing machine partshaving complicated shapes with high dimensional precision and is thuscapable of significantly decreasing the production costs of the machineparts. Therefore, various machine parts produced by applying the powdermetallurgical technology are used in many fields. Further, in recentyears, the requirement for miniaturization or weight lightening ofmachine parts has increased, and various raw material powders for powdermetallurgy for producing small and lightweight machine parts havingsufficient strength have been investigated. For example, JapaneseUnexamined Patent Application Publication No. 1-219101 (Patent Document1), Japanese Unexamined. Patent Application Publication No. 2-217403(Patent Document 2), and Japanese Unexamined Patent ApplicationPublication No. 3-162502 (Patent Document 3) disclose raw materialpowders for powder metallurgy produced by adhering an alloying powder tosurfaces of a pure iron powder or alloy steel powder with a binder(referred to as “segregation-free treatment”). Such powders mainlycomposed of iron (referred to as an “iron-based powder” hereinafter) areusually produced by adding an additive powder (e.g., a copper powder, agraphite powder, an iron phosphide powder, a manganese sulfide powder,or the like) and a lubricant (e.g., zinc stearate, aluminum stearate, orthe like) and the resultant mixed powders are supplied to production ofmachine parts.

As the pure iron powder or alloy steel powder used as a raw material ofthe iron-based powder, there are an atomized iron powder, a reduced ironpowder, and the like according to the production methods. Here, a pureiron powder may be referred to as an iron powder, but the term “ironpowder” in the classification by production methods is used in a broadsense including an alloy steel powder. Hereinafter, the term “ironpowder” represents an iron powder in the broad sense. The alloy steelpowder includes steel powders other than prealloys, i.e., a partiallyalloyed steel powder and a hybrid alloyed steel powder,

However, the iron-based powder, the additive powder, and the lubricanthave different characteristics (i.e., the shape, particle size, and thelike), and thus flowability of a mixed powder is not uniform. Therefore,the following problems (a) to (c) occur:

(a) The iron-based powder, the additive powder, the lubricant, and thelike locally unevenly distribute due to the influence of vibration ordropping during transport of the mixed powder to a storage hopper. Thedeviation due to differences in flowability cannot be completelyprevented even by the segregation-free treatment.

(b) Since relatively large spaces are produced between particles of themixed powder charged in the hopper, the apparent density of the mixedpowder decreases.

(c) The apparent density of the mixed powder depositing in a lowerportion of the hopper increases over time (i.e., due to the influence ofgravitation), while the mixed powder in an upper portion of the hopperis stored at a low apparent density. Therefore, the apparent density ofthe mixed powder is nonuniform in the upper and lower portions of thehopper.

It is difficult to mass-produce machine parts having uniform strengthusing such a mixed powder.

In order to solve the above problems (a) to (c), it is necessary toincrease flowability of the mixed powder of the iron-based powder, theadditive powder, and the lubricant.

Therefore, Japanese Unexamined Patent Application Publication No.5-148505 (Patent Document 4) discloses an iron-based powder mainlycomposed of an iron powder having a predetermined range of particlediameters. However, this technique not only decreases the yield of theiron powder because an iron powder out of the specified range cannot beused but also bears difficulty in uniformly and sufficiently fillingthin-walled cavities, such as a gear edge or the like, with theion-based powder.

On the other hand, US Patent Publication No. U.S. Pat. No. 3,357,818(Patent Document 5) discloses, as means for improving flowability of ametallurgical powder, a technique of adding finest grained inorganiccompounds, particularly oxide compounds (preferably having a particlediameter of 1 or less), in an amount of about 25% of an organiclubricant. Examples of the inorganic compounds include silic acid,titanium dioxide, zirconium dioxide, silicon carbide, iron oxide(Fe₂O₃), and the like.

In addition, Japanese Unexamined Patent Application Publication(Translation of PCT Application) No. 2002-515542 (Patent Document 6)discloses a technique for improving flowability of an iron powder forpowder metallurgy by adding 0.005 to 2% by mass of a metal oxide, suchas SiO₂ of less than 500 nm or the like. Also, this publicationintroduces, as segregation-free treatment, a wet method using a resinsuch as cellulose or the like as a binder (a method of adhering a binderin a natural liquid state or a solvent solution state to an iron powderand then removing liquid contents such as a solvent and the like) anddescribes that a method of dry-mixing the metal oxide after the removalof a liquid content is preferred.

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

However, as a result of examination, the inventors newly found thefollowing: That is, some of various fine particles (for example, SiO₂)described in Patent Publication No. U.S. Pat. No. 3,357,818 (PatentDocument 5) and Japanese Unexamined Patent Application Publication(Translation of PCT Application) No. 2002-515542 (Patent Document 6)frequently decrease mechanical properties of sintered compacts, and itis undesirable to add such fine particles in a blind way.

The present invention aims at solving the above-mentioned problems.Namely, an object of the invention is to provide an iron-based powderfor powder metallurgy which is excellent in flowability and capable ofuniformly filling a thin-walled cavity and which does not decreasemechanical properties of sintered compacts.

In addition, as a result of examination, the inventors newly found thefollowing: It is practically difficult to sufficiently mix finestparticles added for improving flowability so that the finest particlesfunction on the most part of an iron powder. Therefore, a conventionalmethod does not fully utilize the ability of a flowability-improvingagent.

Accordingly, in a further preferred embodiment of the present invention,an object is to resolve the problems and provide a method for producingan iron-based powder which satisfactorily exhibits the effect of aflowability-improving agent and also provide an iron-based powder.

Means for Solving the Problem

The present invention is as follows.

(1) An iron-based powder for powder metallurgy characterized inincluding iron powder with surfaces to each of whichflowability-improving particles adhere through a binder.

The iron powder is an iron powder in the broad sense including an alloysteel powder. The binder may adhere at least a portion of an additivepowder (particularly, an alloying powder) to the iron powder.

(2) The iron-based powder for powder metallurgy of the inventiondescribed above in (1), wherein the iron powder contains less than 50%by mass of an iron powder not having the binder.

For example, when a first iron powder is subjected to segregation-freetreatment and then mixed with a second iron powder not subjected tosegregation-free treatment, the second iron powder corresponds to an“iron powder not having the binder”.

(3) The iron-based powder for powder metallurgy of the inventiondescribed above in (1) or (2), wherein the surfaces of the iron powderare previously treated with a wettability-improving agent to improvewettability with the binder.

Specifically, the sentence “the surfaces of the iron powder are treatedwith a wettability-improving agent to improve wettability with thebinder” represents that the iron powder surfaces are coated with thewettability-improving agent to such an extent that awettability-improving effect is exhibited.

(4) The iron-based powder for powder metallurgy of the inventiondescribed above in any one of (1) to (3), wherein the melting point ofthe flowability-improving particles is 1800° C. or more, and theflowability-improving particles are not sintered with each other duringsintering of an iron-based powder compact.

The flowability-improving particles preferably include at least oneselected from TiO₂, Al₂O₃, ZrO₂, Cr₂O₃, and ZnO, and the averageparticle diameter of the flowability-improving particles is preferablyin a range of 5 to 500 nm.

(5) The iron-based powder for powder metallurgy of the inventiondescribed above in any one of (1) to (4), wherein theflowability-improving particles include PMMA and/or PE, and the averageparticle diameter of the flowability-improving particles is in a rangeof 5 to 500 nm.

Both the flowability-improving particles described above in (4) and theflowability-improving particles described above in (5) may be addedtogether.

(6) The iron-based powder for powder metallurgy of the inventiondescribed above in any one of (1) to (5), wherein the binder is at leastone selected from zinc stearate, lithium stearate, calcium stearate,stearic acid monoamide, and ethylenebis(stearamide).

(7) The iron-based powder for powder metallurgy of the inventiondescribed above in any one of (1) to (6), wherein the iron powder is anatomized iron powder and/or a reduced iron powder.

(8) The iron-based powder for powder metallurgy of the inventiondescribed above in any one of (1) to (7), wherein theflowability-improving particles are contained at a ratio of 0.01 to 0.3parts by mass relative to 100 parts by mass of the iron powder.

(9) A method for producing an iron-based powder containing at least aniron powder and flowability-improving particles, the method including astep of adhering at least a binder to at least a portion of the ironpowder (referred to as “raw material powder A”), a step of mixing theflowability-improving particles with part of a material powder of theiron-based powder without adding a binder (referred to as “raw materialpowder B”), and a step of adding and mixing the raw material powder B(mixture of part of a material powder of the iron-based powder and theflowability-improving particles) with the raw material powder A (ironpowder having the binder adhered thereto).

(10) A method for producing an iron-based powder characterized inincluding a step of adhering at least a binder to a first iron powder, astep of mixing flowability-improving particles with a second iron powderto which a binder is not adhered, and a step of subsequently mixing thefirst iron powder with the second iron powder (containing theflowability-improving particles).

The invention described above in (10) is the most preferred embodimentof the invention described above in (9). A typical example of “a step ofadhering at least a binder” to at least a portion of the iron powder ora first iron powder is segregation-free treatment. Therefore, at leastpart of an additive powder (particularly, an alloying powder) may beadhered to the iron powder by the treatment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view showing an example of appearance of aniron-based powder of the present invention.

FIGS. 2A, 2B, and 2C are electron microscope photographs (“Good”,“Poor”, and “None”, respectively) showing examples of evaluation of adegree of adhesion of flowability-improving particles to surfaces ofiron-based powder.

FIG. 3 is a perspective view schematically showing a principal portionof a filling tester.

REFERENCE NUMERALS

-   -   1 atomized iron powder    -   2 flowability-improving particle    -   11 cavity    -   12 iron-based powder    -   13 filling shoe    -   14 vessel    -   15 moving direction

BEST MODE FOR CARRYING OUT THE INVENTION

A preferred embodiment of the present invention is described below.Except for a portion concerning mixing of flowability-improvingparticles, known powders for powder metallurgy (including selection ofraw materials and additives) and production methods therefor (includingprocedures and apparatuses) (disclosed in, for example, JapaneseUnexamined Patent Application Publication No. 2005-232592, etc) can beapplied.

(Method of Producing Iron-Based Powder)

First, an iron powder and an alloy component are mixed together with abinder under heating using a mixer to produce an iron-based powder forpowder metallurgy (a type of segregation-free treatment).Flowability-improving particles are added after the segregation-freetreatment and are mixed in a dry state with a mixer.

Here, other additives such as a cutting ability improving agent and thelike may be added together with an alloy component and may be mixedunder heating together with a binder. The additives are generallypowders of about 1 to 20 μm. The alloy component is typically a graphitepowder, a Cu powder, a Ni powder, a Cr powder, a W powder, a Mo powder,a Co powder, or the like. The cutting ability improving agent istypically a MnS powder, a CaF₂ powder, a phosphate powder, a BN powder,or the like. In addition, a lubricant having a higher melting point thanthe heating temperature may be added at the same time as the alloycomponent.

Further, after the segregation-free treatment, a powder lubricant ispreferably added for further securing compactability (referred to as a“free lubricant”). Each lubricant can be appropriately selected fromknown lubricants. The flowability-improving particles are preferablyadded and mixed with the iron powder (iron-based powder) after thesegregation-free treatment at the same time as the free lubricant.

As the mixer, a high-speed mixer which is a mechanical mixing-type mixeris preferred from the viewpoint of mixing force. However, the mixer maybe appropriately selected according to the production amount of theiron-based powder, desired flowability, and the like.

Specific procedures include charging a predetermined amount of ironpowder in a high-speed mixer, and adding the alloy component such as agraphite powder, a Cu powder, or the like and the binder. After theseraw materials are charged, heating and mixing is started. The rotationalspeed of a rotating impeller in the high-speed mixer depends on the sizeof a mixing tank, and the shape of the rotating impeller, but isgenerally preferably about 1 to 10 m/sec in terms of the peripheralspeed at the tip of the rotating impeller. Heating and mixing isperformed until the temperature in the mixing tank is the melting pointof the binder or higher, and mixing is performed at a temperature of themelting point or higher for about 1 to 30 minutes. After the rawmaterials are sufficiently mixed, the mixing tank is cooled. When thebinder is solidified in the cooling step, additives such as the alloycomponent and the like are adhered to the surfaces of the iron powder.

The binder may be appropriately selected from known binders, and any oneof a heat melting type and a type of being melted by heating and thensolidified by cooling can be used. In particular, a binder havinglubricity after solidification is preferred.

The reason for this is that this type decreases frictional force betweenpowder particles, improves flowability of a powder, and promotesrearrangement of particles at an early stage of compaction.Specifically, metallic soap, amide wax, polyamide, polyethylene,polyethylene oxide, or the like is used. In particular, zinc stearate,lithium stearate, calcium stearate, stearic acid monoamide, andethylenebis(stearamide) are preferred. These binders may be used aloneor in a mixture of two or more. The preferred adding amount is about0.05 to 0.8 parts by mass relative to 100 parts by mass of iron powder.

Meanwhile, as the iron powder, there are various iron powders accordingto the production methods, but a water atomized iron powder or a reducediron powder is preferably used in view of compactability,characteristics of a compacted body, and characteristics of a sinteredbody. Such an iron powder has irregularity in particle surfaces, and thestrength of a compacted body and sintered body is increased due toengagement of irregularity during powder compaction. The iron powder isnot particularly limited as long as it is in the above-definedcategories, i.e., a pure iron powder or an alloy steel powder (includinga partially alloyed steel powder and a hybrid alloyed steel powder). Thepure iron powder contains 98% or more of iron and impurities as thebalance. The alloy steel powder contains alloy components such as Mn,Cu, Mo, Cr, W, Ni, P, S, V, Si, and the like in a total of about 10% orless. In addition, previous addition of an alloy composition to moltensteel is referred to as “prealloying”, bonding of particles containingalloy components to iron powder surfaces by diffusion reaction isreferred to as “partial alloying”, and combination of prealloying andpartial alloying is referred to as “hybrid alloying”.

The particle diameter of an iron powder is generally in a range of 60 to100 μm in terms of average particle diameter (according to sieveanalysis defined by Japan Powder Metallurgy Association standard JPMAP02-1992). (Wettability-improving treatment with wettability-improvingagent)

The binder is molten at a melting point or higher so that particlesurfaces of a raw material powder in a mixing tank are wetted with thebinder. Since the water atomized iron powder and the reduced iron powderhave irregularity on the surfaces thereof, the binder tends to locallystay in the irregularity. Therefore, the binder nonuniformly distributeson the surfaces of the iron powder. In order to make the binderdistribution uniform, it is necessary to improve wettability of ironpowder surfaces with the binder. Therefore, it is preferred to use awettability-improving agent for improving wettability of iron powdersurfaces with the binder.

An effective method of treatment with the wettability-improving agent isa method of previously coating at least iron powder surfaces with thewettability-improving agent before the segregation-free treatment(before heat-mixing of the binder, the iron powder, and other alloycomponents). When a silane coupling agent is used, the silane couplingagent (liquid) may be added to the iron powder charged in a mixing tank,followed by stirring at room temperature for about 1 to 10 minutes.Then, the binder and the other alloy components are charged andheat-mixed. The preferred coating amount is about 0.005 to 0.1 parts bymass relative to 100 parts by mass of iron powder.

Other conceivable wettability-improving agents include an acethyleneglycol surfactant and a polyhydric alcohol surfactant. Both agents areliquid, and the treatment method and proper coating amount are the sameas the silane coupling agent. However, the stirring conditions may becontrolled according to the wettability-improving agent used. As amixing device, a device with high mixing force (mixing speed) ispreferably used, and for example, a rotor mixer such as a Henschelmixer, a high-speed mixer, or the like, or a mixer having mixing forceequivalent to that of such a mixer is preferred.

(Flowability-Improving Particles)

The flowability-improving particles used in the present invention arecomposed of fine powder having the effect of improving flowability ofthe atomized iron powder. In the present invention, in consideration ofthe viewpoint that the mechanical properties of sintered compacts arenot decreased, types of the flowability-improving particles are roughlydivided into the following two:

(A) particles having a melting point of 1800° C. or more (preferablyinorganic compounds, particularly inorganic oxides, specifically atleast one of TiO₂, Al₂O₃, ZrO₂, Cr₂O₃, and ZnO, and most preferablyTiO₂); and

(B) at least one of polymethyl methacrylate (PMMA) and polyethylene(PE).

It is generally known that if fine irregularity is present on surfacesof powder particles, the contact area between the particles isdecreased, thereby decreasing adhesive force between the particles.Although the water atomized iron powder and reduced iron powder alsohave irregularity in the surfaces, the irregularity is not sufficientfor decreasing adhesive force because the curvature is 0.1 to 50 μm⁻¹and relatively small. By adhering the flowability-improving particles tothe iron powder surfaces, the adhesive force between the particles canbe sufficiently decreased.

However, some fine particles decrease the mechanical properties ofsintered compacts, such as strength and toughness (SiO₂ and the like),and not all types of fine particles can be used. As a result ofresearch, the inventors found that particles belonging to theabove-described group (A) or (B) do not decrease the mechanicalproperties of sintered compacts. The inventors estimate the reason whythese particles do not decrease mechanical properties as follows.

Since particles having a melting point of less than 1800° C. are meltedor softened by sintering (about 900° C. to 1400° C.), the particles aresupposed to be deformed at an acute angle in conformity with the gapsbetween the particles, thereby enhancing the adverse effect on themechanical properties. On the other hand, as in the group (A), when themelting point is 1800° C. or more, the particles are thought to maintaina state close to the initial (relatively) spherical shape, therebycausing no adverse effect on the mechanical properties. The group (B)consists of organic substances which are thought to disappear due todecomposition during sintering, thereby causing little adverse effect onthe mechanical properties.

In addition, in the group (A), inorganic substances, particularlyoxides, are preferred because substances having high melting points areeasily available. Also, it is determined from the results of experimentsand examination that in the group (A), at least one of TiO₂, Al₂O₃,ZrO₂, Cr₂O₃, and ZnO, particularly TiO₂, is preferred. Further, it isdetermined from the results of examination of particle diameters,hardness, and the like that in the group (B), particularly PMMA and PEamong organic substances are preferred.

The flowability-improving particles are adhered to the iron powderthrough the binder. In order to adhere finest grained particles to otherparticles by sufficiently dispersing the particles, generally,procedures of dispersing the finest grained particles in a liquid tocoat the particles with the liquid and then evaporating the liquid arerequired. However, as a result of research in the present invention, itwas found that flowability can be sufficiently decreased by adding thebinder to the iron powder and then dry-mixing the finest grainedparticles to adhere the finest grained particles to the iron powderthrough the binder. This is possibly due to the following facts.

-   -   The flowability-improving particles easily adhere to the surface        of the binder.    -   Exposed portions of the binder most degrade flowability with        other particles, and in order to improve flowability, it is        highly effective to impart projections to the surfaces of the        binder using particles.

In the method of the present invention, the above-exemplified binderwhich is heat-melted for coating is more preferred than other binders(for example, a binder which is dissolved in a solvent for coating).This is because the heat-melting type binder exhibits a strong force ofadsorbing flowable particles.

The average particles diameter of the flowability-improving particles ispreferably 5 nm or more. When the average particle diameter of theflowability-improving particles is less than 5 nm, the particles may beburied in irregularity of the surfaces of the iron powder or in thelubricant present on the surfaces of the iron powder. These fineparticles are present as aggregates, but when the particles areexcessively fine, the particles undesirably adhere while staying inaggregates to the surfaces of the iron powder. In addition, theproduction cost of fine particles generally increases as the particlediameter decreases.

The average particles diameter of the flowability-improving particles ispreferably 500 nm or less. When the average particle diameter exceeds500 nm, the diameter is the same as the curvature of irregularityoriginally present in the surfaces of the iron powder, and thus themeaning of intended adhesion of the particles is significantlydecreased. In particular, the flowability-improving particles of above(A) are present in a sintered body without decomposition duringsintering. The particles can be regarded as an inclusion in steel, andwhen the particles are excessively large, strength of a sintered body isdecreased. The average particle diameter is more preferably 100 nm orless.

For these reasons, the average particle diameter of theflowability-improving particles is preferably in the range of 5 to 500nm. As the particle diameter of the flowability-improving particles, avalue determined by BET specific surface measurement on the assumptionthat the shape of the particles is spherical is used for (A), and avalue measured by a microtrack method using ethanol as a dispersionmedium is used for (B).

In order to obtain the significant effect of improving flowability, theamount of the flowability-improving particles added is preferably 0.01parts by mass or more relative to 100 parts by mass of the iron powder.The amount is more preferably 0.05 parts by mass or more. On the otherhand, the amount of the flowability-improving particles added ispreferably 0.3 parts by mass or less relative to 100 parts by mass ofthe iron powder. When the amount exceeds 0.3 parts by mass, incompaction under the same pressure, the density of a green compactdecreases, and consequently, strength of a sintered body undesirablydecreases. The amount is more preferably 0.2 parts by mass or less.

Therefore, the amount of the flowability-improving particles added ispreferably in a range of 0.01 to 0.3 parts by mass relative to 100 partsby mass of the iron powder.

The effect of addition of the flowability-improving particles is thatfine irregularity is provided in the surfaces of the iron powder todecrease the contact area between particles, thereby decreasing adhesiveforce. There is also the effect of inhibiting adhesion between thebinder and the binder present on the surfaces of the iron powder. FIG. 1is a schematic view showing an example of the iron-based powder of thepresent invention. FIG. 1 indicates that the flowability-improvingparticles disperse and adhere to the surfaces of atomized iron powder 1.In addition, it was confirmed by a C distribution and an oxide metalelement distribution obtained by EPMA that the binder is present in aportion where the flowability-improving particles adhere.

(Addition of Iron Powder not Having Binder)

In another mode of the present invention, the iron-based powder containsan iron powder not having the binder. Considering the above-mentionedfunction principle of the flowability-improving particles, the ironpowder not having the binder adhering thereto is considered to haveexcellent flowability. This mode is based on the above-describedviewpoint, and the iron powder contains less than 50% by mass of an ironpowder not having the binder. Such an iron-based powder can be preparedby mixing an iron powder not subjected to segregation-free treatmentwith an iron powder subjected to segregation-free treatment. The averageparticle diameter range of the iron powder preferred for addition is thesame as the above-described general iron powder.

The amount of the iron powder (having uncoated surfaces) not having thebinder on the surfaces is less than 50% by mass relative to the whole ofthe iron powder. When amount of the iron powder not having the binder is50% by mass or more, ejection force increases during compaction, and insome cases, die galling phenomenon may occur, and/or defects may occurin a compacted body. The amount of the iron powder not having the binderis more preferably 20% by mass or less. The amount is preferably 5% bymass or more from the viewpoint of achieving a significant effect, andmore preferably 10% by mass or more.

Further, as an unexpected effect, the flowability-improving particlesare first mixed with the iron powder not having the binder and thenmixed with the iron powder having the binder (i.e., after thesegregation-free treatment), thereby further improving flowability.Although the reason for this is not elucidated, a supposed reason isthat the flowability-improving particles more uniformly disperse on theentire surface of the binder due to the aggregation preventing effectthat aggregates of the flowability-improving particles are ground by theiron powder with uncoated surfaces.

This mechanism is expected when the particles not having the binder arereplaced by another material powder not having the binder (for example,an alloying powder such as a Cu powder or the like, a cutting abilityimproving powder, or the like). Namely, a similar effect is obtained bymixing the flowability-improving particles with part of a raw materialpowder of the iron-based powder, which is not limited to an iron powder,without adding the binder (for example, referred to as “raw materialpowder B”) and then adding and mixing the raw material powder B with aniron powder subjected to segregation-free treatment (referred to as “rawmaterial powder A”). Of course, the raw material powder used for the rawmaterial powder B is not limited to one type and may contain wholeamount of a certain additive powder.

As the particles not having the binder in the raw material powder B, aniron powder is most preferably used. This is because of the advantagethat the mass of particles and the amount of particles added can beincreased to enhance grinding force, and unlike other raw materialpowders, there is no possibility of segregation even if the binder isnot used.

(Other)

The content of a composition (contained as an alloy steel powder andadhering with the binder) other than iron in the iron-based powder ofthe present invention is 10 parts by mass or less relative to 100 partsby mass of iron powder. When the iron-based powder of the presentinvention is applied to powder metallurgy, additive powders (an alloyingpowder, a cutting ability improving powder, and the like) may be furtheradded and mixed for controlling the composition and the like of asintered body before filling in a die and compaction molding.

EXAMPLE Example 1

Each of the binders shown in Table 1, and an iron powder, a graphitepowder, a Cu powder, and the like shown in Table 1 were heat-mixed witha Henschel-type high-speed mixer. Then, the resultant mixture was cooledto 60° C., and flowability-improving particles and a free lubricantshown in Tables 1 and 2 were added and mixed. The physical properties ofthe flowability-improving particles were as shown in Table 3. In some ofthe samples (Nos. 12 and 13), an iron powder previously subjected towettability-improving treatment with a silane coupling agent(phenyltrimethoxy silane) under the above-described preferred conditionswas used.

The surfaces of each of the resultant iron-based powders were observedwith a scanning electron microscope (SEM) to evaluate the adhesion stateof the flowability-improving particles. FIGS. 2A to 2C show examples ofphotographs taken for the surfaces of the iron-based powders togetherwith the results of evaluation. In FIG. 2A, ◯ (Good) indicates asatisfactory state in the present invention, and in FIG. 2B and FIG. 2C,Δ (Poor) and × (None) indicate unsatisfactory states, respectively.

The filling performance of each of the resultant iron-based powders wasevaluated with a filling test machine shown in FIG. 3. In evaluation, acavity 11 provided in a vessel 14 and having a length of 20 mm, a depthof 40 mm, and a width of 0.5 mm was filled with the iron-based powderfrom a filling shoe 13. The filling shoe 13 filled with the iron-basedpowder was moved in an arrowed moving direction 15 shown in FIG. 3 at amoving rate of 200 mm/sec and maintained above the cavity 11 for aretention time of 0.5 seconds. The percentage of filling density(filling weight/cavity volume) after filling to the apparent densitybefore filling is determined as the filling rate (filling rate of 100%represents complete filling). The same test was repeated 10 times, andfilling variation was represented by a standard deviation of fillingrates. The results are shown in Table 2.

In addition, a mold was filled with each of the iron-based powders andcompressed (compaction pressure 686 MPa) to form into a shape of tensilespecimen having a thickness of 5 mm. Further, sintering (sinteringtemperature 1130° C., sintering time 20 minutes) was performed in a RXgas atmosphere to form a tensile specimen. The results of a tensile testare also shown in Table 2.

Any one of the invention examples shows a good adhesion state of theflowability-improving particles and good filling variation. Also,strength of sintered bodies is good.

When TiO₂ was used as the flowability-improving particles under the sameconditions as the above, the filling variation can be minimized. It isfound that by performing wettability-improving treatment, strength of asintered compact is improved, and flowability is slightly improved as awhole.

In No. 17 in which the flowability-improving particles were not addedand in No. 18 in which the flowability-improving particles were notsufficiently adhered to the iron powder surfaces, the filling variationis large.

Further, in No. 20 using as the flowability-improving particles SiO₂having a melting point of 1450° C., flowability is good, but strength ofa sintered compact is significantly decreased.

TABLE 1 Wettability- improving agent (parts Binder (parts by mass*¹)Free lubricant (parts by mass*¹) Iron-based powder (parts by mass)*¹ bymass*¹) Stearic Stearic Other Silane acid Ethylenebis Zinc Ethylenebisacid Zinc No. 301A*² 255M*³ Graphite (powder) coupling agent amide(stearamide) stearate (stearamide) amide stearate Remarks 1 97.4 — 0.6Cu: 2 — 0.3 0.3 — — — 0.2 Example 2 97.4 — 0.6 Cu: 2 — 0.3 0.3 — — — 0.2Example 3 97.4 — 0.6 Cu: 2 — 0.3 0.3 — — — 0.2 Example 4 97.4 — 0.6 Cu:2 — 0.3 0.3 — — — 0.2 Example 5 97.4 — 0.6 Cu: 2 — 0.3 0.3 — — — 0.2Example 6 97.4 — 0.6 Cu: 2 — 0.3 0.3 — — — 0.2 Example 7 97.4 — 0.6 Cu:2 — 0.3 0.3 — — — 0.2 Example 8 97.4 — 0.6 Cu: 2 — 0.3 0.3 — — — 0.2Example 9 87.4 10.0 0.6 Cu: 2 — — — 0.4 — — 0.4 Example 10 77.4 20.0 0.6Cu: 2 — — — 0.4 — — 0.4 Example 11 — 97.4 0.6 Cu: 2 — — — 0.4 — — 0.4Example 12 97.4 — 0.6 Cu: 2 0.05 0.3 0.3 — — — 0.2 Example 13 97.4 — 0.6Cu: 2 0.05 0.3 0.3 — — — 0.2 Example 14 97.4 — 0.6 Cu: 2 — 0.2 0.2 — 0.10.1 0.2 Example 15 97.4 — 0.6 Cu: 2 — 0.2 0.2 —  0.15  0.15 0.1 Example16 97.4 — 0.6 Cu: 2 — — — 0.4 — — 0.4 Example 17 97.4 — 0.6 Cu: 2 — 0.30.3 — — — 0.2 Comp. Example 18 97.4 — 0.6 Cu: 2 — 0.3 0.3 — — — 0.2Comp. Example 19 97.4 — 0.6 Cu: 2 — 0.3 0.3 — — — 0.2 Comp. Example 2097.4 — 0.6 Ni: 2 — 0.3  0.35 — — —  0.15 Example 21 Alloy steel 0.8 Cu:1 — — — — — — — Example powder*⁴: 98.2 22 97.4 — 0.6 Cu: 2 — — — 0.4 — —0.4 Example 23 97.4 — 0.6 Cu: 2 — 0.3 0.3 — 0.1 0.1 — Example 24 77.4SGM10CU- 0.6 — — 0.3 0.3 — — — 0.2 Example 304*⁵: 20 —: Not added*¹Value relative to 100 parts by mass of iron powder + alloy (graphite,Cu, Ni, Mo) powders (97.4% (in No. 2, 98.2%) of a value relative to 100parts by mass of iron powder) *²JIP(TM) 300A: atomized iron powdermanufactured by JFE Steel Corporation, average particle diameter 70 to90 μm *³JIP(TM) 255A: reduced iron powder manufactured by JFE SteelCorporation, average particle diameter 70 to 90 μm *⁴Atomized ironpowder pre-alloyed with 0.45% by mass of Mo, average particle diameter70 to 90 μm *⁵SGM10CU-304: atomized iron powder to which 10% by mass ofCu was diffused and bonded

TABLE 2 Sintered compact tensile Flowability-improving particles (partsby mass)*¹ Filling strength No. TiO₂ Al₂O₃ ZrO₂ Cr₂O₃ ZnO PMMA PE SiO₂Evaluation*² variation (MPa) Remarks 1  0.05 — — — — — — — Good 0.2 425Example 2 0.1 — — — — — — — Good 0.1 420 Example 3 0.2 — — — — — — —Good 0.3 410 Example 4 — 0.1  — — — — — — Good 0.2 425 Example 5 — — 0.2— — — — — Good 0.3 410 Example 6 — — — 0.1 0.1 — — — Good 0.2 425Example 7 — — — — — 0.1 — — Good 0.2 430 Example 8 — — — — — — 0.1 —Good 0.3 430 Example 9 0.1 — — — — — — — Good 0.3 430 Example 10 0.1 — —— — — — — Good 0.3 430 Example 11 0.1 — — — — — — — Good 0.2 430 Example12 0.1 — — — — — — — Good 0.1 425 Example 13 — 0.05 — — — — — — Good 0.2427 Example 14 0.1 — — — — — — — Good 0.3 430 Example 15 0.1 — — — — — —— Good 0.2 425 Example 16 0.1 — — — — — — — Good 0.3 427 Example 17 — —— — — — — — None 2.0 430 Comp. Example 18  0.005 — — — — — — — Poor 1.8420 Comp. Example 19 — — — — — — — 0.2 Good 0.3 380 Comp. Example 20 0.05 0.05 — — — — — — Good 0.2 700 Example 21 — — — — — 0.1 0.1 — Good0.3 600 Example 22  0.05 — — — —  0.05 — — Good 0.3 425 Example 23  0.020.02 — — — —  0.02 — Good 0.3 420 Example 24 0.1 — — — — — — — Good 0.2425 Example —: Not added *¹Value relative to 100 parts by mass of ironpowder + alloy (graphite, Cu, Ni, Mo) powders (97.4% (in No. 2, 98.2%)of a value relative to 100 parts by mass of iron powder) *²Visualevaluation of an adhesion state of flowability-improving particles in aSEM image

TABLE 3 BET Average Flowability- specific particle improving TradeDensity AD (apparent surface diameter Single particle Melting pointparticles Manufacturer name (Mg/m³) density (Mg/m³) (m²/g) (μm) diameter(nm) (° C.) TiO₂ Ishihara A-100 3.7-3.9 0.2 237.2 0.2 6 1800 SangyoKaisha, Ltd. Al₂O₃ Nippon Aerosil Alu C 4.0 0.05 100 13 2300 Co., Ltd.ZrO₂ Hakusui Tech F-3 6.0 0.1 20 0.1 50 3000 Co., Ltd. Cr₂O₃ 5.2 2400ZnO Hakusui Tech F-3 5.7 0.1 20 0.1 50 2000 Co., Ltd. PMMA Zeon KaseiCo., F325 1 0.4 18.5 25 50 — Ltd. PE 1 5 100 — SiO₂ Cabot SpecialtyCAB-O- 2.2 0.016 299.1 0.2-0.3 9 1450 Chemicals Inc. SIL EH-5 Blank:Unconfirmed

Example 2

Each of the binders shown in Table 4, and an iron powder, a graphitepowder, a Cu powder, and the like shown in Table 4 were heat-mixed witha Henschel-type high-speed mixer. Then, the resultant mixture was cooledto 60° C., and a free lubricant and flowability-improving particlesshown in Table 5 were added and mixed. In Nos. 31 to 33 and 36 to 40,the flowability-improving particles were previously mixed with an ironpowder not having a binder and then mixed with an iron powder having abinder adhering thereto (the iron powder heat-mixed and then cooled to60° C. as described above), while in Nos. 34 and 35, theflowability-improving particles and the iron powder not having thebinder were separately mixed with an iron powder having a binderadhering thereto without previous mixing. In No. 40, an iron powder towhich the binder was added was subjected to wettability-improvingtreatment as in Example 1.

Then, the same examination was in Example 1 was performed. The resultsare shown in Table 5. The adhesion state of the flowability-improvingparticles by a scanning electron microscope (SEM) was determined as(Good) in all samples.

Any one of the invention examples showed good filling performance. Incomparison under the same conditions, when the flowability-improvingparticles were previously mixed with an iron powder not having thebinder (Nos. 31 and 32), the filling performance was obviously improvedas compared with the case in which the flowability-improving particlesand the iron powder not having the binder were separately added (Nos. 34and 35).

TABLE 4 Wettability- Iron-based powder improving Iron powder (withoutBinder) (with binder) agent (parts (parts by mass*¹) (parts by mass)*¹by mass*¹) Flowability- Binder (parts by mass*¹) Other Silane couplingimproving Stearic Ethylenebis Zinc No. 301A*² 255M*³ Graphite (powder)agent 301A*² 255M*³ particles*⁵ acid amide (stearamide) stearate Remarks31 97.4 — 0.6 Cu: 2 — — 5.0 Mixing 0.2 0.2 — Example 32 77.4 — 0.6 Cu: 2— — 20.0  Mixing 0.2 0.2 — Example 33 57.4 — 0.6 Cu: 2 — 40.0 — Mixing —— 0.4 Example 34 92.4 — 0.6 Cu: 2 — — 5.0 Separately 0.2 0.2 — Example35 77.4 — 0.6 Cu: 2 — — 20.0  Separately 0.2 0.2 — Example 36 92.4 — 0.6Cu: 2 — — 5.0 Mixing — — 0.4 Example 37 97.4 — 0.6 Cu: 1 — — 5.0 Mixing0.2 0.2 — Example Ni: 1 38 Alloy steel 0.6 — — — 5.0 Mixing 0.2 0.2 —Example powder*⁴: 94.4 39 92.4 — 0.6 Cu: 2 — — 5.0 Mixing 0.2 0.2 —Example 40 92.4 — 0.6 Cu: 2 0.05 — 5.0 Mixing 0.2 0.2 — Example —: Notadded *¹Value relative to 100 parts by mass of iron powder + alloy(graphite, Cu, Ni) powders (97.4% (in No. 38, 99.4%) of a value relativeto 100 parts by mass of iron powder) *²JIP(TM) 300A: atomized ironpowder manufactured by JFE Steel Corporation, average particle diameter70 to 90 μm *³JIP(TM) 255A: reduced iron powder manufactured by JFESteel Corporation, average particle diameter 70 to 90 μm *⁴Atomized ironpowder pre-alloyed with 2 parts by mass of Cu, average particle diameter70 to 90 μm *⁵Mixing: The flowability-improving particles werepreviously mixed with iron powder not having a binder. Separately: Theflowability-improving particles were separately added without beingpreviously mixed.

TABLE 5 Free lubricant (parts Sintered by mass)*¹ Flowability-improvingcompact Ethylene- Stearic particles (parts by tensile bis acid Zincmass)*¹ Filling strength No. (stearamide) amide stearate TiO₂ PMMA PEOther variation (MPa) Remarks 31 0.1 0.1 0.2 0.1 — — — 0.1 420 Example32 0.15 0.15 0.1 — — — Al₂O₃: 0.05 0.2 427 Example 33 — — 0.4 0.1 — — —0.3 430 Example 34 0.15 0.15 0.1 0.1 — — — 0.2 420 Example 35 0.15 0.150.1 — — — Al₂O₃: 0.05 0.3 420 Example 36 — — 0.4 0.15 — — — 0.1 410Example 37 0.1 0.1 0.2 0.05 0.02 0.02 — 0.2 650 Example 38 0.1 0.1 0.2 —0.1  — — 0.3 420 Example 39 0.1 0.1 0.2 0.05 — — ZrO₂, Cr₂O₃, ZnO: 0.3420 Example each 0.05 40 0.1 0.1 0.2 0.1 — — — 0.2 420 Example —: Notadded *¹Value relative to 100 parts by mass of iron powder + alloy(graphite, Cu, Ni) powders (97.4% (in No. 38, 99.4%) of a value relativeto 100 parts by mass of iron powder)

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to produce aniron-based powder containing an iron powder as a material, havingexcellent flowability, and being suitable for use in powder metallurgywithout decreasing the mechanical properties of sintered compacts.

1. An iron-based powder for powder metallurgy comprising iron powderwith surfaces to each of which flowability-improving particles adherethrough a binder.
 2. The iron-based powder for powder metallurgyaccording to claim 1, wherein the iron powder contains less than 50% bymass of an iron powder not having the binder.
 3. The iron-based powderfor powder metallurgy according to claim 1, wherein the surfaces of theiron powder are previously treated with a wettability-improving agent toimprove wettability with the binder.
 4. The iron-based powder for powdermetallurgy according to claim 1, wherein the melting point of theflowability-improving particles is 1800° C. or more.
 5. The iron-basedpowder for powder metallurgy according to claim 4, wherein theflowability-improving particles include at least one selected from TiO₂,Al₂O₃, ZrO₂, Cr₂O₃, and ZnO, and the average particle diameter of theflowability-improving particles is in a range of 5 to 500 nm.
 6. Theiron-based powder for powder metallurgy according to claim 1, whereinthe flowability-improving particles include PMMA and/or PE, and theaverage particle diameter of the flowability-improving particles is in arange of 5 to 500 nm.
 7. The iron-based powder for powder metallurgyaccording to claim 1, wherein the binder is at least one selected fromzinc stearate, lithium stearate, calcium stearate, stearic acidmonoamide, and ethylenebis(stearamide).
 8. The iron-based powder forpowder metallurgy according to claim 1, wherein the iron powder is anatomized iron powder and/or a reduced iron powder.
 9. The iron-basedpowder for powder metallurgy according to claim 1, wherein theflowability-improving particles are contained at a ratio of 0.01 to 0.3parts by mass relative to 100 parts by mass of the iron powder.
 10. Amethod for producing an iron-based powder containing at least an ironpowder and flowability-improving particles, the method comprising: astep of adhering at least a binder to at least a portion of the ironpowder; a step of mixing the flowability-improving particles with partof a material powder of the iron-based powder without adding a binder;and a step of adding and mixing a mixture of part of a material powderof the iron-based powder and the flowability-improving particles withthe iron powder having the binder adhered thereto.
 11. A method forproducing an iron-based powder comprising: a step of adhering at least abinder to a first iron powder; a step of mixing flowability-improvingparticles with a second iron powder; and a step of subsequently mixingthe first iron powder with the second iron powder.